Exemplary embodiments of the present invention will be described hereafter with particular reference to a bias controller for a Mach-Zehnder modulator (often known simply as an MZ modulator), the controller and modulator being employed in a communications system to modulate an input optical carrier signal with a radio frequency (RF) communications signal. Whilst the teachings of the present invention have great utility in optical communications systems, that is to say communications systems where the nodes of the system are optically connected, it will be immediately appreciated by persons of ordinary skill in the art that the teachings of the invention may otherwise be applied. Accordingly, the following illustrative description should not be read as being limited solely to communications systems or indeed to MZ modulators.
With the above in mind, reference will now be made to FIG. 1 where an illustrative schematic representation is provided of a known MZ modulator 1 of the type that is oft employed in optical communications systems.
MZ modulators provide a mechanism whereby an input optical carrier signal may be modulated with a communications signal, for example with an RF communications signal. In this example the modulator is effectively an interferometer, created by forming an optical waveguide in a suitable substrate such as Lithium Niobate (LiNbO3) or Gallium Arsenide (GaAs) or Indium Phosphide (InP). The waveguide 11 of the modulator depicted in FIG. 1 is split into two branches 11a, 11b before recombining at an optical coupler 13. An optical carrier signal in the form of a beam of light enters the modulator from the left of FIG. 1 and exits the modulator from the right of FIG. 1 having passed through both branches of the waveguide.
As shown in FIG. 1, one of the waveguide branches 11a includes an asymmetry 15 that functions to introduce a phase difference between light travelling down respective branches 11a, 11b of the waveguide 11. The phase difference is chosen to be approximately 90 degrees at the wavelength of operation, which is typically in the region of 1300 or 1550 nanometers—this induces quadrature bias where the optical output is nominally 50% of its' maximum.
Lithium Niobate (in common with other similar materials such as GaAs or InP) is a glass-like material with a crystal structure that exhibits an electro-optic effect whereby the refractive index of the crystal structure changes as a voltage is applied thereto. In particular, the direction of the electric field induced by the applied voltage causes an increase or decrease in refractive index—an increased refractive index acting to slow light travelling through the crystal, and a decreased refractive index acting to increase the speed of light travelling through the crystal. In MZ modulators, the Lithium Niobate material is usually arranged so as to have an X cut, Y propagate crystal orientation with respect to the input optical signal, and in this context an electric field applied in the X direction (positive or negative) causes a change in the refractive index of the material that affects the speed of the light passing along the Y axis.
As shown in FIG. 1, a modulating electrode 7 is provided between the branches 11a, 11b of the waveguide, and when this electrode is energized by an applied signal (for example by a radio frequency or digital communications signal), positive and negative electric fields are established between the modulating electrode 7 and, respectively, first 3 and second 5 ground planes. The modulating electrode is designed as a transmission line so that the modulating signal travels with the optical carrier signal through the modulator, thereby enabling high modulating frequencies to be achieved.
The positive and negative electric fields cause the refractive index of the two branches 11a, 11b of the waveguide 11 to change (the positive field causing an increase in refractive index for branch 11a, and the negative field causing a decrease in refractive index for branch 11b), and the resulting different propagation speeds of the optical carrier signal through each branch cause a change in phase in the signals output to the optical combiner 13, which phase change causes the output level of light from the optical combiner 13 to change. In effect, as the electric fields experienced by each branch varies with the communications signal applied to the modulating electrode, so the phase difference between light passing through the two branches changes and the output level of the optical signal output from the combiner 13 varies accordingly. The net effect of this is that the input optical carrier signal is modulated with the communications signal applied to the modulating electrode 7.
Referring now to FIG. 2 of the accompanying drawings, the overall transfer characteristic of the modulator is approximately sinusoidal, with the most linear modulation being achieved in and around the quadrature point (also known simply as “quadrature”, both terms being used interchangeably herein), namely the point where there is a 90° phase relationship between light travelling through respective branches 11a, 11b of the waveguide. The transfer function is a repeating function, and as such there are many quadrature points at different bias voltages but all with the same power output. Indicated in FIG. 2 is a first quadrature point A where the output power is increasing with voltage bias, and hence this quadrature point is referred to as a positive slope quadrature bias point. Also indicated are two further quadrature points B and C where the output power is decreasing with voltage bias, and these quadrature points are each referred to as negative slope quadrature bias points.
Whilst the foregoing sounds eminently achievable, in practice this 90° phase shift is rarely if ever achieved, and to compensate for this it is usual to include a biasable component 9, and to apply a DC bias voltage to the biasable component 9, to return the modulator to or near one of the aforementioned quadrature points. In the arrangement depicted in FIG. 1, the biasable component includes a discrete bias electrode, but this is merely illustrative as a number of alternative arrangements are known to persons skilled in the art. For example, a bias voltage may be applied directly to the modulating electrode by a so-called bias-Tee. In such an arrangement, the DC bias is coupled to the electrode via an inductor, and the applied signal (for example an RF communications signal) is coupled to the electrode via a capacitor.
A problem with this arrangement that has to be addressed is that the bias point, i.e. the voltage that needs to be applied to the biasable component to return the modulator to or near the quadrature point, shifts over time. For example, so-called trapped charges in the waveguide and temperature variations can each cause the bias point to shift at a rate of anything from a few millivolts per hour to several volts per hour.
Once consequence of this is that it is generally not possible to provide a system where the bias voltage, once set, need not be changed, and as such it is usual to provide some sort of dynamic bias control to enable modulator linearity to be maintained over an extended period of time.
In the analogue domain dynamic bias control has previously been achieved by applying a pilot tone (for example a 10 kHz tone for a GHz communications signal of interest) to the modulating electrode, by monitoring the output of the modulator and by adjusting the bias voltage based on that output. For example, as the 2nd harmonic of the pilot tone usually tends to zero at or around the quadrature point, one previously proposed approach monitors this second harmonic and adjusts the applied DC bias voltage to drive the second harmonic to zero. A similar approach has previously been proposed for the digital domain, but in this instance the signal applied is typically a square wave dither signal, and the output is monitored by a digital signal processor.
Whilst each of these approaches do enable a form of dynamic bias control to be provided, they each have attendant disadvantages. For example, the application of a pilot tone necessarily gives rise to modulation products (for example sidebands) that limit the achievable dynamic range of the system, and for high-fidelity optical links this reduction in dynamic range is simply unacceptable. In very high-speed links (for example, digital links with speeds of up to 100 GBit/s and analogue links with frequencies of up to 60 GHz), the application of a dither can adversely affect the achievable data rate and the length of link that is achievable. Another disadvantage particularly prevalent in instances where multiple channels are required, for example in a phased array antenna system, is that as each modulator is different the bias control hardware needs to be fully replicated for each and every modulator. This increases system bulk, complexity and cost.
It would be advantageous if a bias controller could be devised that at least mitigated, and preferably avoided, problems of this ilk.